While many of us either take it for granted or don’t give it a second thought, ice matters — especially in cocktails. It influences the concentration, flavor, and temperature of our mixed drinks. The large, clear cubes that are commonplace at craft cocktail bars and on Instagram represent the pinnacle of frozen water. Lots of bartenders spend hours making ice to produce not just the best-looking drinks, but the best-quality cocktails they can. The process is surprisingly simple, if time-consuming, and can easily be adapted at home.

Before the how, though, here’s why: Large ice melts at a slower rate than standard cubes popped from a tray or dispensed by a fridge, meaning less dilution. This, in turn, keeps cocktails cooler for longer. When frozen in large blocks, ice can also be carved into specific shapes and customized with individual glassware and drinks in mind. For example, in the case of the Whiskey Highball, singular ice spears not only keep the drink refreshingly chilled but help to maintain the cocktail’s fizz.

Although pre-made silicone molds designed specifically to freeze large cubes exist, anyone who owns one will know the ice never turns out crystal clear. Beyond aesthetics, there are solid reasons for putting in the extra effort to make clear ice. The process works as a form of filtration, pushing out the cloudy mass of solid impurities and trapped air that typically forms in the center of standard cubes.

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Clear Ice 101

The method for making clear ice relies on “directional freezing,” a technique first popularized by drinks writer Camper English in 2009 on his blog, Alcademics. There, English has created something of a shrine to the process of clarifying frozen H2O, with numerous explainer articles and experiments devoted to topics such as “Distilled vs. Tap Water” and “Melting and refreezing water.”

English explains that conventional ice trays form cubes with a cloudy center because the water is being frozen at the same rate on all six surfaces. This pushes water’s trapped air and impurities into the center of the cube where they are frozen solid. (If you have some ice cubes on hand, you’ll note that the edges for each appear clear.) His method for making clear ice works instead by slowing the rate of cooling on five out of the six sides of the cube, using an insulated container such as a picnic cooler.

By leaving one side (the top) exposed, the water freezes down vertically, pushing the cloudiness to the bottom of the container. If allowed to freeze solid, the bottom quarter or third of the block of ice will end up cloudy. But if the cooler and cube are removed from the freezer beforehand, the impurities will remain unfrozen in the still-liquid portion, while the frozen ice will be crystal clear.

What You Need to Make Clear Ice

English’s directional freezing method requires little more than an insulated picnic cooler, but there are certain factors to consider if you’re going to buy one new, as well as fancy tools to facilitate the process.

First and foremost, you’ll want to make sure the cooler will fit in your freezer. As a rough guide, a 5-quart cooler, which tends to be the smallest available, yields more than half a dozen 2-by-2-inch cubes.

Another factor to check prior to purchasing is whether the cooler contains a removable lid. Those that don’t will take up more space in the freezer as the process relies on the water being left uncovered, and may even prevent the ice from being removed, as one unlucky member of the r/clearice subReddit discovered.

Otherwise, all that’s needed are the tools to cut and shape the large block into smaller pieces. Sophisticated solutions abound on professional bar supply websites, including picks, chippers, chisels, and saws. But it’s easy to get by with just a large cutting board, some clean dish towels, a serrated knife, and a wooden mallet or hammer.

How to Make Clear Ice

Start by filling your cooler with water and place it inside the freezer with the lid removed. Filtered water will subtly improve things on the flavor front, but won’t impact the clarity of the final ice. The popular idea that boiling water prior to freezing also has little to no impact on the final clarity.

After 24 hours, or once the majority of the water appears to have frozen, remove the cooler and leave it out on a countertop at room temperature. If the bottom section of the ice appears cloudy at this point, all the water has likely frozen solid. This doesn’t mean you have to start the process again, but will require some extra steps later down the line.

In the meantime, set up a workstation for breaking down the ice into smaller cubes. Line a large cutting board with a clean kitchen towel, and have the serrated knife and mallet on hand nearby. If possible, work over or next to a sink as things are about to get wet.

When it seems like the ice has become unstuck from the sides of the cooler (around five minutes), turn it over onto the towel and chopping board. The block should release easily, and if the mass hasn’t frozen completely solid, will be followed by some water. Tidy up any protruding edges or shards using the mallet, leaving a solid, clear block of ice.

Now it’s time to break it down into smaller pieces. Measure the diameter of your rocks or Highball glass, and subtract a few millimeters to arrive at the perfect width for your cubes or rectangular prisms. Cut slowly into the ice with the serrated knife and when the incision is at least 1 centimeter deep, tap down on the top of the blade using the hammer. Inserting a food-safe stainless steel chisel instead of a knife admittedly makes this process easier, but either way, a few firm knocks will do the job.

This step should then be repeated until all the ice has been processed into your desired shapes. In cases where the entire block froze in the cooler, the cloudy mass (now) on the top side of the upturned ice will need to be removed. Rather than throwing this section away, keep it in the freezer and use it to stir or shake cocktails. And always store all your ice in resealable plastic bags or containers to ensure cubes don’t pick up food odors from their surroundings.

Finally, when the time comes to serve a drink with your clear ice, remove it from the freezer and allow it to temper for a minute or two before mixing. This extra step helps to avoid any cracking, which can occur when the ice comes into contact with liquids because of the temperature differences. Then enjoy the fruits of your labor, making sure to marvel at your perfectly clear cube of ice until it’s gone.